Most printing processes cannot directly vary the density of a printing substance, or pigment, that is deposited on a print medium. Instead, they vary the density of the image by restricting the area of the pigment on the print medium. In this way, the native color of the print media and the pigment combine to produce the desired density. To be effective, the scale of this pattern of pigmented and non-pigmented medium must be small enough such that the observer does not see the pattern and instead integrates the pattern into an image that has variable density. This process is called screening or dithering.
Digital printing systems typically use dithering to produce images with variable density. Before the image is printed, a source image is converted to a dithered image. Because the source image has variable density, each pixel of the source image can take on many values, typically represented by a two-dimensional array of eight- or ten-bit values per image component. In the case of a color image, there may be four image components: cyan, magenta, yellow, and black (CMYK).
Dithering reduces the image to pixel values with two values, on or off. To retain the original resolution of the source image a dithered image may distribute the pigment dots over a finer grid than the original resolution of the source image. In the simplest case, if the source image has a range of X density values, then the resolution of the dithered image may be X times the resolution of the source image. Depending on how the pigmentation process interacts with the print medium, this expansion can be less than X times the source image resolution.
Currently, dithering is done with the knowledge that the process of printing will naturally orient the rectilinear print path to the grid of the source image. Furthermore, current dithering operations are based on an expectation that the printing process will cause the dither dots to be selected from the image in a stereotyped pattern that will ensure all the dither dots will be printed in a predictable time.